(1) Field of the Invention
The present invention relates to a laser emitting apparatus for use as light source with laser applied machinery such as a laser beam printer, a laser COM (computer output microfilming) system and the like.
More particularly, the invention relates to a laser emitting apparatus comprising a semiconductor laser, a laser drive device for applying an excitation current to the semiconductor laser, photoreceptor means for receiving a laser beam emitted from the semiconductor laser and outputting a signal corresponding to the intensity of the laser beam, and control means operable in response to the signal output from the photoreceptor means for controlling the excitation current so that the laser beam has a predetermined intensity.
(2) Description of the Prior Art
The laser beam printer and laser COM system utilizing the laser beam emitted from the semiconductor laser carries out recording with the laser beam directly modulated on the basis of image information. In order to carry out recording faithful to the image information, therefore, it is necessary to maintain a constant laser beam intensity at all times. Since the intensity of the laser beam emitted from the semiconductor laser is easily influenced by the temperature, feedback control is effected on the laser beam emitted from the semiconductor laser as noted above.
The laser beam emission raises the temperature of the semiconductor laser. On the other hand, it is known that the excitation current to be applied to the semiconductor laser for causing the semiconductor laser to emit a laser beam of a certain intensity depends on the semiconductor laser temperature, and the higher temperature requires a greater quantity of excitation current.
It often is the case with the laser applied machinery noted above that the laser beam is not emitted at all times but is emitted only when necessary, for example, only when recording is carried out. The semiconductor laser temperature begins to lower gradually when the laser beam emission is broken such as upon completion of recording after a continuous emission of the laser beam. After the laser beam emission ceases, the laser drive device retains the excitation current as received at the time of emission stoppage.
Consequently, when the laser beam emission is resumed, the excitation current is temporarily applied to the semiconductor laser in a greater quantity than is necessary for causing the laser beam emission even though the semiconductor laser has a low temperature. This is likely to deteriorate or damage the semiconductor laser.
In order to avoid such an inconvenience, a proposal has been made to cause the semiconductor laser to emit the laser beam at fixed intervals when the semiconductor laser does not emit the laser beam continuously, and to control the quantity of the excitation current by using the output from the photoreceptor means.
According to the above proposal, however, the laser beam must be emitted at fixed intervals during a waiting time when recording is not carried out and a continuous emission of the laser beam is unnecessary. It inevitably results in an elongated operating time of the semiconductor laser, which could well shorten the life of the semiconductor laser.